Asthma is a complex disease involving the concerted actions of multiple inflammatory and immune cells, spasmogens, inflammatory mediators, cytokines and growth factors. In recent practice there have been four major classes of compounds used in the treatment of asthma, namely bronchodilators (e.g., beta-adrenoceptor agonists), anti-inflammatory agents (e.g., corticosteroids), prophylactic anti-allergic agents (e.g., cromolyn sodium) and xanthines (e.g., theophylline) which appear to possess both bronchodilating and anti-inflammatory activity.
Theophylline has been a preferred drug of first choice in the treatment of asthma. Although it has been touted for its direct bronchodilatory action, theophylline's therapeutic value is now believed to also stem from anti-inflammatory activity. Its mechanism of action remains unclear. However, it is believed that several of its cellular activities are important in its activity as an anti-asthmatic, including cyclic nucleotide phosphodiesterase inhibition, adenosine receptor antagonism, stimulation of catecholamine release, and its ability to increase the number and activity of suppressor T-lymphocytes. While all of these actually may contribute to its activity, only PDE inhibition may account for both the anti-inflammatory and bronchodilatory components. However, theophylline is known to have a narrow therapeutic index, and a wide range of untoward side effects which are considered problematic.
Of the activities mentioned above, theophlylline's activity in inhibiting cyclic nucleotide phosphodiesterase has received considerable attention recently. Cyclic nucleotide phosphodiesterases (PDEs) have received considerable attention as molecular targets for anti-asthmatic agents. Cyclic 3',5'-adenosine monophosphate (cAMP) and cyclic 3',5'-guanosine monophosphate (cGMP) are known second messengers that mediate the functional responses of cells to a multitude of hormones, neurotransmitters and autocoids. At least two therapeutically important effects could result from phosphodiesterase inhibition, and the consequent rise in intracellular (cAMP) or (cGMP) in key cells in the pathophysiology of asthma. These re smooth muscle relaxation (resulting in bronchodilation) and anti-inflammatory activity.
It has become known that there are multiple, distinct PDE isoenzymes which differ in their cellular distribution. A variety of inhibitors possessing a marked degree of selectivity for one isoenzyme or the other have been synthesized.
The structure-activity relationships (SAR) of isozyme-selective inhibitors has been discussed in detail, e.g., in the article of Theodore J. Torphy, et al., "Novel Phosphodiesterase Inhibitors For The Therapy Of Asthma", Drug News & Prospectives, 6(4) May 1993, pages 203-214. The PDE enzymes can be grouped into five families according to their specificity toward hydrolysis of cAMP or cGMP, their sensitivity to regulation by calcium, calmodulin or cGMP, and their selective inhibition by various compounds. PDE I is stimulated by Ca.sup.2+ /calmodulin. PDE II is cGMP-stimulated, and is found in the heart and adrenals. PDE III is cGMP-inhibited, and inhibition of this enzyme creates positive inotropic activity. PDE IV is cAMP specific, and its inhibition causes airway relaxation, anti-inflammatory and antidepressant activity. PDE V appears to be important in regulating cGMP content in vascular smooth muscle, and therefore PDE V inhibitors may have cardiovascular activity.
While there are compounds derived from numerous structure activity relationship studies which provide PDE III inhibition, the number of structural classes of PDE IV inhibitors is relatively limited. Analogues of rolipram, which following structural formula: ##STR1## and of Ro-20-1724, which has the following structural formula: ##STR2## have been studied.
Rolipram, which was initially studied because of its activity as an antidepressant has been shown to selectively inhibit the PDE IV enzyme and this compound has since become a standard agent in the classification of PDE enzyme subtypes. There appears to be considerable therapeutic potential for PDE IV inhibitors. Besides initial work suggesting an antidepressant action, rolipram has been investigated for its anti-inflammatory effects, particularly in asthma. In-vitro, rolipram, Ro-20-1724 and other PDE IV inhibitors have been shown to inhibit (1) mediator synthesis/release in mast cells, basophils, monocytes and eosinophils; (2) respiratory burst, chemotaxis and degranulation in neutrophils and eosinophils; and (3) mitogen-dependent growth and differentiation in lymphocytes (The PDE IV Family Of Calcium-Phosphodiesterases Enzymes, John A. Lowe, III, et al., Drugs of the Future 1992, 17(9):799-807).
Other PDE-IV inhibitors are 3,8-alkyl-disubstituted-6-thioxanthines disclosed by U.S. Pat. No. 4,925,847, issued May 15, 1990 to Hofer, the disclosure of which is incorporated by reference herein in its entirety.
PDE IV is present in all the major inflammatory cells in asthma including eosinophils, neutrophils, T-lymphocytes, macrophages and endothelial cells. Its inhibition causes down-regulation of cellular activation and relaxes smooth muscle cells in the trachea and bronchus. On the other hand, inhibition of PDE III, which is present in myocardium, causes an increase in both the force and rate of cardiac contractility. These are undesirable side effects for an anti-inflammatory agent. Theophylline, a non-selective PDE inhibitor, inhibits both PDE III and PDE IV, resulting in both desirable anti-asthmatic effects and undesirable cardiovascular stimulation. With this well-known distinction between PDE isozymes, the opportunity for concomitant anti-inflammatory and bronchodilator activity without many of the side effects associated with theophylline therapy is apparent. The increased incidence of morbidity and mortality due to asthma in many Western countries over the last decade has focused the clinical emphasis on the inflammatory nature of this disease and the benefit of inhaled steroids.
Additional thioxanthine compounds are known to the art. However, although some have been suggested to be useful for treating, e.g., asthma, the specific anti-PDE IV activity of these compounds has not been determined. For example, French Patent No. 188M, issued on Aug. 12, 1960 to May & Baker, Ltd, discloses the synthesis of the disubstituted thioxanthines 3-butyl-1-methyl-6-thioxanthine and 3-isobutyl-1-methyl-6-thioxanthine for bronchial or coronary artery dilation without disclosing any PDE IV inhibitory effects. French Patent No. 188M also discloses trisubstituted 6-thioxanthines (Formula I of the 188M patent) having at the 1 and 3 positions an alcohol or alkyl (C.sub.1-6), straight or branched and H or an alcohol (C.sub.1-6) at the 8 position.
Woolridge et al., 1962, J. Chem. Soc. Annex IV: 1863-1868 discloses the synthesis of disubstituted 6-thioxanthines: 1,3 and 3,7-disubstituted 6-thioxanthines for bronchial or coronary dilation as well as 1,3,8 lower tri-alkyl substituted 6-thioxanthines where the alkyl groups are methyl or ethyl. PDE IV activity was uncharacterized.
Armitage et al., 1961, Brit. J. Pharm. 17:196-207, disclose trisubstituted 6-thioxanthines having bronchial and coronary dilator activity. The 1,3,8-trisubstituted 6-thioxanthines disclosed by Armitage are 1,3,8-trimethyl-6-thioxanthine and 1,3-dimethyl-8-ethyl-6-thioxanthine.
Some trisubstituted xanthine derivatives having diuretic, renal protective and vasodilator properties are disclosed by U.S. Pat. No. 5,068,236, issued to Suzuki et al. on Nov. 26, 1991. Suzuki et al. disclose xanthines, including trisubstituted xanthines having a lower alkyl independently at positions 1 and 3 and a --CH.sub.2 --(R.sup.4)R.sup.5 at the 8 position, wherein R.sup.4 and R.sup.5 are independently substituted or unsubstituted alicyclic alkyl or substituted or unsubstituted aryl. The exemplified trisubstituted compounds having bronchial and coronary dilator activity are not characterized as to PDE IV activity.
Therefore, there remains a continuing need to find new thioxanthine compounds having more selective and improved PDE IV inhibitory activity.